Store the butter in an opaque container to prevent oxidation.
Light accelerates the breakdown of fat in butter at room temperature; a dark container preserves its flavor and quality.
Butter keeps perfectly well at room temperature due to its high fat and low water content. However, exposure to light can accelerate fat breakdown and oxidation, meaning it will spoil faster. Light is deceptive, and when it comes into contact with the fat and water in butter, it triggers a reaction that causes it to lose quality and flavor. While butter technically doesn't become unsafe to eat at room temperature, its flavor begins to deteriorate the longer it's exposed.
The easiest way to get the best of both worlds, room-temperature butter that retains its high quality, is to store it in an opaque container that completely covers it. This minimizes light exposure while still allowing you to enjoy this salty, spreadable ingredient. It's important to avoid storing butter in a metal container. Due to the composition of most metals, this storage method accelerates butter oxidation, causing it to deteriorate quickly. A porcelain butter dish or a dark, airtight container is the best option.